This invention relates to an optical fiber sensor to be used in an optical rotary encoder, optical limit switch and the like.
Heretofore an optical fiber sensor to be used in a rotary encoder and the like has been constructed as shown in FIG. 1 including two optical fibers 1 and 2 of 1 mm diameter. The end portions of the optical fibers 1 and 2 are joined together such that peripheral surfaces thereof are in line contact with each other, and held at a position where the end surfaces of the optical fibers are spaced apart from a slit 3 by a predetermined distance Z, and arranged in side by side along the moving direction A of the slit 3. A luminous element (not shown) such as LED is coupled with the optical fiber 1, while a light receiving element (not shown) such as a phototransistor is coupled with the optical fiber 2.
With the above described arrangement of the conventional optical fiber sensor, the light emitted from the luminous element is transmitted through the optical fiber 1 in a direction B to irradiate reflecting surfaces 3a and nonreflecting surfaces 3e of the slit 3. One part of light reflected from the reflecting surfaces 3a enters the optical fiber 2 to be transmitted in a direction C toward the light receiving element. Since the slit 3 is constantly shifted in direction A, the light receiving element detects a high level H.sub.o and a low level L.sub.o of light quantity alternately in accordance with the variation of relative positions of the surfaces 3a and 3b to the end portion of the optical fiber sensor.
Ordinarily the high level H.sub.o and the low level L.sub.o of light quantity varies according to the distance Z between the end portion of the optical fiber sensor and the slit 3. FIG. 3 depicts the relation between the output of the light receiving element and the distance Z in case of utilizing optical fibers of 0.75 mm diameter and an encoder in the form of a circular disc for the slit 3. When the difference between the high level H.sub.o and low level L.sub.o is large, the stability of the output of the sensor is improved and the detection of a smaller width in the slit is made possible.
As is apparent in FIG. 3, in the conventional optical fiber sensor, the increasing rate of the high level H.sub.o according to an increase in the distance Z is less than the increasing rate of the low level L.sub.o under the same condition. In the conventional construction utilizing optical fibers of a circular cross section, the maximum light emitting angle and the maximum light receiving angle of the fibers are selected to be approximately 30.degree.. As a consequence, an effective area from where light is reflected effectively coincides with an overlapping area P.sub.o (slashed in FIG. 2) between an irradiating region X.sub.o from the fiber 1 and a light receiving region Y.sub.o of the fiber 2. Thus the high level H.sub.o in FIG. 3 does not increase sufficiently according to the increase in the distance Z because the effective reflecting area is comparatively small and only a small part of the reflecting surfaces 3a is overlapped by the light receiving region Y.sub.o. On the other hand, the light quantity of low level L.sub.o at the time of detecting low level light increases rapidly in accordance with the increase of the distance Z because an increase in the distance Z increases the width of the overlapping portion P.sub.o such that one part of the reflecting surfaces 3a located on both sides of the nonreflecting surfaces 3e are brought into the right and left ends of the overlapping portion P.sub.o as viewed in FIG. 2, and the light reflected from the part is also received in the optical fiber 2.
In case where a slit to be detected having a small width is read out by use of two optical fibers as described above, it is essential from the view point of subsequent electric processes that a large difference exists between the high level and low level of light quantities. Accordingly, the conventional optical fiber sensor which exhibits an insufficient light difference regardless of an increased distance Z could not provide a sufficient resolution.